Choosing the right circuit-board material can be a frustrating experience. After all, every manufacturer of printed-circuit-board (PCB) materials promises outstanding performance for their products, offering an array of dielectric formulations to achieve improved stability, or power-handling capability, or lower loss. Wouldn’t it be simpler to choose one material, such as FR-4, for all applications? In an ideal world, a single PCB material could serve all applications. But requirements for two applications may vary so widely, that no single PCB material can provide optimum performance for both.
Previous posts examined some of the key material parameters pertaining to high-frequency laminates, such as dielectric constant, thermal conductivity, coefficient of thermal expansion (CTE), and even flexibility when used in conformal circuits. But how does an engineer combine all this information about a material’s electrical and mechanical properties when trying to choose the perfect substrate for a particular application? It can be a complex process, but it may be possible to simplify that process.
Perhaps any search for suitable PCB material for a given application should start with the application itself. It is often a customer’s application, and its particular requirements, that drive particular material formulations for optimum performance in one type of application, such as a high-frequency, high-power amplifier. The better an engineer understands their target application, the easier the process is for selecting a circuit-board laminate for that application. Defining which performance parameters are the most critical for an application can help guide an engineer to the best choice of circuit-board laminate.
Defining laminate specifications
A high-frequency circuit design may have a list of required specifications that can fill a page or more, but typically a handful of those specifications are the critical ones that call for special design or fabrication approaches. For a PCB laminate, that can provide the foundation for meeting the most critical requirements. Where a bandpass filter is defined by such parameters as center frequency, percentage bandwidth, rejection, and passband insertion loss, a laminate is characterized by a completely different set of parameters, such as dielectric constant, thermal coefficient of dielectric constant, and CTE. Even the thickness of a PCB laminate can impact the high-frequency performance of a circuit. So, evaluating a particular application to help choose the right circuit board material is a matter of finding which of the application’s key performance parameters relate to which of the PCB laminate’s characteristics, and what possible tradeoffs may exist.
Defining the needs of an application can be as simple as a series of “filtering” processes, sorting by means of larger issues and working down to performance tradeoffs. For example, will a PCB laminate ultimately be used in a military system, for commercial use, for industrial use, in space, for a medical application, or in a combination of these application areas? Knowing where the circuit-board material is going, such as in a military electronic-warfare (EW) system, will eliminate some PCB materials from consideration, since they won’t meet the basic electrical and mechanical requirements for military use. Of course, if a designer is hoping to sell their circuit across commercial and military markets, the PCB material must be suitable for military environments. For circuit-board materials that will be used for products across multiple market areas, the market with the most rigorous set of requirements (usually military or aerospace) will set the requirements for the PCB material.
Reviewing the requirements
Reviewing the requirements of an application can also help to define necessary and unnecessary tradeoffs. One of the more obvious tradeoffs is cost versus performance. Compared to a high-performance PTFE-based laminate, FR-4 can save a bundle. But it may not provide the high-frequency performance needed for an application, and it may not provide much frequency or amplitude stability even if it does reach the right frequency. Even within such an obvious tradeoff are finer points for comparison: part of the overall price of using a given laminate includes processing costs—some materials are simpler and less expensive to process than others, depending upon the composition of the laminate. Circuit size can also contribute to lowering costs. Choosing a laminate with a higher dielectric constant can yield more circuits per laminate panel, provided that the electrical effects of the higher dielectric constant are acceptable for that application. These and other factors must be considered when making a “simple” tradeoff evaluation between costs versus performance for different laminate materials.
Most high-frequency laminate specifiers start with relative dielectric constant when comparing products from different suppliers, and then check other parameters, such as dissipation loss and CTE. Laminate manufacturers specify their products with a specific value and some amount of variation, such as 3.48 ± 0.05 in the z-direction at 10 GHz for our RO4350B™ laminate. But as noted in an earlier post about applying a dielectric constant, this may not be the best value to use in a computer simulation. Choosing the right laminate material requires confidence in how the material has been characterized, so that simulations will represent final results. A future post will detail some of the methods that laminate suppliers use to determine material parameters such as dielectric constant, typically by fabricating a circuit structure with known characteristics on the PCB material.
Coming up next
I will go into greater detail on how different PCB laminate specifications relate to the performance levels of different high-frequency circuits. For example, for a high-power microwave amplifier, a laminate’s thermal conductivity will certainly be one of the first parameters to compare among different substrates under consideration. But if a laminate has a high dissipation factor, it contributes to high circuit insertion loss. The higher loss results in more heat generated through the amplifier circuit, in turn requiring higher thermal conductivity. In this example, these two parameters (and possibly others) must be balanced and compared from laminate to laminate to make the best choice for a particular power amplifier circuit. To be continued ………
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